A common way of providing telephone subscribers with services that are not provided by the basic telephone station set is to incorporate them in an adjunct to the station set. One such adjunct is an automatic dialer. The adjunct dialer, which is typically connected in series between the associated telephone station set and the central office, often includes only the circuitry that is necessary to provide the dialing function. The adjunct dialer relies upon the associated telephone station set to provide the means for going off-hook and for acquiring dial tone from the central office. Thus when properly used, the adjunct dialer should be activated only after the telephone subcriber has removed the handset from the switchhook of the associated telephone station set and has recognized that dial tone has been returned from the central office.
A problem arises from this arrangement when the controls of the adjunct are operated before the handset is removed from the switchhook, loop current from the central office has been established through the adjunct, and dial tone has been returned. Automatic dialing will proceed unchecked until the entire telephone number has been outpulsed. If the user tries to correct the situation by removing the handset from the switchhook contacts, the telephone line will be seized by the continued outpulsing. As a result, the central office will respond by providing dial tone and by attempting to record the erroneously outpulsed digits. Alternatively, if the adjunct has been activated to dial the wrong telephone number and the user wishes to stop the automatic dialing, he must wait until the completion of dialing before starting a new operation. Thus it is believed apparent that a circuit which is able to detect switchhook status during dial pulsing is required so that continued dial pulsing may be immediately and automatically aborted whenever the associated telephone set is on-hook. The need for such control becomes more apparent if the automatic dialing is slow as in pulse dialing as opposed to multi-frequency tone dialing.
One solution to this problem is known, namely to transmit switchhook status information over a separate lead to the adjunct equipment. This lead proceeds from an extra contact of the associated telephone set's switchhook to the adjunct telephone station equipment. Off-hook or on-hook status information is provided over this lead to the adjunct equipment. Accordingly, the adjunct dialer is able to abort dialing if its controls are inadvertently operated when the telephone set is on-hook. There are then required in this embodiment at least three leads: the standard tip lead and ring lead, and an additional switchhook status lead. Hohmann et al., U.S. Pat. No. 3,128,351, which discloses an adjunct repertory dialer, is exemplary of apparatus which employ the additional lead for this purpose.
While this method provides a solution, it is desirable today, with the trend toward telephone station equipment modularity, to employ just two leads, the standard tip and ring leads, between the various items of station apparatus and to employ standard plug and jack arrangements to tie them together. In this manner, efficiencies in telephone equipment inventory control are achieved, and the subscriber is encouraged to install and to modify his telephone equipment arrangement without the aid of telephone company personnel. Exemplary plug and jack arrangements currently in use are described in Hardesty, U.S. Pat. No. 3,860,316, and its progeny and Krumreich et al., U.S. Pat. No. 3,850,497, and its progeny.
While it is desirable to provide a remote switchhook status check function without increasing the number of leads normally provided, it is likewise recognized that, in so doing, it is undesirable to increase the impedance of the telephone subscriber loop. Such an increase limits both the transmission and signaling distances from the central office where a telephone station set may be placed.
Another solution which is known is to provide a manually operated button for the adjunct dialer. The user of the dialer recognizing his mistake may cancel the dial pulsing by pushing the button before starting the dialing process anew. This means is undesirable because the burden of correcting the mistake is on the user. It is believed apparent that an automatic abortion of dialing is preferable.
R. L. Cerbone, application Ser, No. 122,431 filed Feb. 19, 1980, now U.S. Pat. No. 4,283,603, discloses an on-hook/off-hook status check circuit for a multifrequency tone dialing system. The Cerbone circuit includes a first opto-isolator connected in series with the subscriber loop and a second opto-isolator connected across the loop on the station set side of the circuit. These opto-isolators are sequentially activated to perform the switchhook status check. The Cerbone circuit, however, provides a means of seizing the line from the station set which is unnecessary in dial pulsing applications. Accordingly, it is preferred to apply the Cerbone circuit with automatic multi-frequency tone dialing systems.
Therefore, despite the above-described solutions, there remains a need for a switchhook status check circuit adapted for use with dial pulse dialing systems. This circuit should check for off-hook status during dial pulsing and abort dial pulsing if the associated telephone set is on-hook.